In order to research the influence of the proportion of large particle size fraction in sintering return fines on the index of sintering production, the physical and chemical properties of sintering return fines are analyzed by chemical analysis, XRD analysis, thermogravimetric analysis, etc. The influence of the proportion of +5 mm particle size fraction on sintering index is studied by using micro sintering experimental device and sintering cup simulation experiment. The results show that the composition of different particle sizes in the return fines is also quite different; as the particle size of the return fines increases, the amount of magnetite and calcium ferrite in the mineral phase increases. The larger the particle size of return fines is, the higher the assimilation temperature and melting temperature are, and as the particle size of return fines gets smaller, it becomes more conducive to the formation of liquid phase. When the proportion of return fines +5 mm is more than 25%, the granulation effect of the mixture becomes worse, the sintering yield and drum strength decrease, the solid fuel consumption increases, and the proportion of 5 ~ 10 mm in the particle size composition of sinter products also increases. Therefore, it is necessary to control the proportion of +5 mm in the return fines less than 25% in the sintering production proportioning process.